Studies will be carried out to determine if polyene antibiotics can, like filipin, exist in sterol-binding and sterol-nonbinding forms. The filipin complex can exist in both forms in aqueous solution. If other binding and nonbinding forms are found, optimum conditions will be determined for obtaining each form. The relative affinity of various polyene antibiotics such as filipin, nystatin, amphotericin B, and pimaricin for sterols will be determined using fluorimetric techniques. Relative quantum efficiency will be used because of its sensitivity, concentration independence, and reliability. The effect of experimental conditions; i.e., pH, ionic strength, nucleotides, buffers, temperature, etc., on the interaction of filipin and pimaricin with biologically important sterols will be determined. A C14-labeled filipin complex will be prepared, and this material will be used to study the binding of filipin to a) soluble proteins such as albumins; b) phospholipid dispersions, and c) membrane-containing systems such as microsomes and mitochondria.